High-temperature electrochemical etching has become a widely used technique for the manufacturing of electronic components using micro-fabrication processes. The electronic components that can be made involving the electrochemical etching process include metal diaphragms for use in sensors, actuators, pressure transducers, etc., and silicon and semiconductor wafers. Because the electrochemical etching process typically involves a very severe environment and requires the workpiece to be placed in long-time exposures to a highly corrosive and strongly acidic or strongly basic etching solution at elevated temperatures, it is necessary to provide a special workpiece holder that will protect the workpiece from the invasion of the etching solution into those portions that are not intended to be etched.
More specifically, because of the extremely harsh environment to which a workpiece is being exposed during the high-temperature electrochemical etching operations, it is desirable to provide a workpiece holder that will allow the etching solution to contact only the intended portions of the workpiece (i.e., those portions that are intended to be etched), and will protect the rear and side portions of the workpiece from any possible contact with the highly corrosive etching solution. Also, because an electric current is required during the electrochemical etching operation, the workpiece holder must provide a protectively sealed environment for an electric conductor wire to go into the workpiece holder and conduct electricity to the workpiece. Furthermore, it also desirable to provide a workpiece holder that can be easily and safely assembled/disassembled and facilitate a satisfactory control of the electrochemical etching operation, even in light of the harsh and difficult conditions under which it is expected to work.
A typical workpiece holder for electrochemical etching operations is shown in FIG. 1. Several problems have been encountered, most notably in that it does not completely prevent the rear and side portion from invasion by the etching fluid. This problem becomes more ostensible during high-temperature electrochemical etching operations. Furthermore, the conventional workpiece holders use a metal plate as the contact electrode to conduct electricity between a conductor wire and the workpiece. If the metal plate contains an unacceptable unevenness on their surface, such unevenness could cause the workpiece to crack when it is urged during the assembly of the etching apparatus. Although it has been proposed to use eutectic In-Ga alloy as the contact electrode to solve the cracking problem, which is caused by the uneven metal surface, and to improve the stability of the contact between the electrode and the workpiece, this is not a very practical approach. One of the problems associated with using the In-Ga alloy as the contact electrode is that, because of it is a liquid phase, it is very difficult to place the contact electrode in a vertical position, thus making the etching process very cumbersome.
One method to solve the leaking problem experienced with conventional workpiece holders is to place two substantially concentric O-rings on the base of the workpiece holder, and introduce nitrogen gas into the space between the two O-rings. This method was taught in Sensors and Actuators A29, pp 209-215 (1991). The nitrogen pressure between the two O-rings prevents the penetration of the etching solution. In electrochemical etching operations, however, because it is necessary to provide an external wire to conduct electric current to the workpiece, this method, which utilizes sealed pressure to prevent leaking, is impractical. Furthermore, this method does not protect the side portions of the workpiece during electrochemical etching operations.
U.S. Pat. No. 4,303,408 teaches an apparatus for selective electrochemical etching. The advantage of this apparatus is that it does not require a conventional workpiece holder and it only allows the area to be etched to be exposed to the etching solution; therefore, it can easily solve the problems associated with the placement of the conductor wire. However, because the apparatus disclosed in the '408 patent involves an open environment, it is difficult to operate at elevated temperatures. Furthermore, the selective electrochemical etching apparatus disclosed in the '408 patent has only limited applications, and is not suitable for large scale productions.